Sulfur is an objectionable element which is nearly ubiquitous in fossil fuels, where it occurs as both inorganic sulfur (mineralized as in iron pyrite) and organic sulfur (covalently bound to carbonaceous molecules). The presence of sulfur has been correlated with corrosion of pipeline, pumping, and refining equipment, and with premature breakdown of combustion engines. Sulfur also contaminates or poisons many catalysts which are used in the refining and combustion of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products such as sulfur dioxide leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as on agricultural areas located downwind of combustion facilities. Monticello and Finnerty (1985), 39 ANN. REV. MICROBIOL. 371-389. Regulations such as the Clean Air Act of 1964 require the removal of sulfur, either pre- or post-combustion, from virtually all fossil fuels. Conformity with such legislation has become increasingly problematic due to the rising need to utilize lower-grade, higher-sulfur fossil fuels as clean-burning, low-sulfur petroleum reserves become depleted, as well as the progressively more stringent reductions in sulfur emissions required by regulatory authorities. Monticello and Kilbane (1990), Practical considerations in biodesulfurization of petroleum, IGT's 3RD INTL. SYMP. ON GAS, OIL, COAL, AND ENV. BIOTECHNOL., New Orleans, La.
There are several well-known physicochemical methods for depleting the sulfur content of fossil fuels prior to combustion. One method that is widely-used for the removal of organic sulfur is hydrodesulfurization (HDS). In HDS, the fossil fuel is contacted with hydrogen gas at elevated temperature and pressure, in the presence of a catalyst. Organic sulfur is removed by the reductive conversion of sulfur bound to carbonaceous molecules to H.sub.2 S, a corrosive gaseous product which is separated from the treated fuel by stripping. As with other desulfurization techniques, HDS is not equally effective in removing all forms of sulfur found in fossil fuels. Gary and Handwerk (1975), PETROLEUM REFINING: TECHNOLOGY AND ECONOMICS (Marcel Dekker, Inc., publ.) 114-120.
For example, HDS is not particularly effective for the desulfurization of coal, wherein inorganic sulfur, especially pyritic sulfur, can constitute 50% or more of the total sulfur content, the remainder being various forms of organic sulfur. The total sulfur content of coal can typically be close to about 10 wt % or it can be as low as about 0.2 wt %, depending on the geographic location of the coal source. Pyritic sulfur is not efficaciously removed by HDS. Thus, only a fraction of the total sulfur content of coal is susceptible to removal by HDS.
HDS is relatively more suitable for desulfurizing petroleum, such as crude oil or refining intermediates thereof, as organic sulfur can account for close to 100% of the sulfur content of these fossil fuels. Crude oils can typically range from close to about 5 wt % down to about 0.1 wt % organic sulfur; crude oils obtained from the Persian Gulf area and from Venezuela can be particularly high in sulfur content. Monticello and Kilbane (1990), Practical considerations in biodesulfurization of petroleum, IGT's 3RD INTL. SYMP. ON GAS, OIL, COAL, AND ENV. BIOTECHNOL., New Orleans, La., and Monticello and Finnerty (1985), 39 ANN. REV. MICROBIOL. 371-389.
Organic sulfur in both coal and petroleum fossil fuels is present in a myriad of compounds, some of which are termed labile in that they can readily be desulfurized, others of which are termed refractory in that they do not easily yield to conventional desulfurization treatment, e.g., by HDS. Shih, S. S. et al. (1990), AIChE Abstract No. 264B (complete text available upon request from the American Institute of Chemical Engineers); hereinafter Shih et al. Thus, even HDS-treated fossil fuels must be post-combustively desulfurized using an apparatus such as a flue scrubber. Flue scrubbers are expensive to install and difficult to maintain, especially for small combustion facilities. Moreover, of the sulfur-generated problems noted above, the use of flue scrubbers in conjunction with HDS is directed to addressing environmental acid deposition, rather than other sulfur-associated problems, such as corrosion of machinery and poisoning of catalysts.
Mercaptans, thioethers, and disulfides exemplify classes of sulfur-containing carbonaceous molecules that are labile to desulfurizing treatments such as HDS. Aromatic carbonaceous molecules, especially those in which sulfur is bonded to the hydrocarbon matrix in aromatic bonds, are refractory to desulfurization by conventional means, e.g., HDS. Such refractory molecules typically require desulfurization conditions harsh enough to degrade valuable hydrocarbons in the fossil fuel. Shih et al. Hence, refractory organic sulfur molecules account for a large proportion of the residual sulfur present in many combustible fuel products.
The foregoing limitations to conventional desulfurization methods such as HDS have spurred considerable and longstanding interest among those engaged in the extraction and refining of fossil fuels in developing commercially viable techniques of microbial desulfurization (MDS). MDS is generally described as the harnessing of metabolic processes of suitable bacteria to the desulfurization of fossil fuels. MDS typically involves mild (e.g., ambient) conditions, and does not involve the extremes of temperature and pressure required for HDS. Several species of chemolithotrophic bacteria have been investigated in connection with MDS development, due to their abilities to consume (catabolize) forms of sulfur that are generally found in fossil fuels. For example, species such as Thiobacillus ferrooxidans are capable of extracting energy from the conversion of pyritic sulfur to water-soluble sulfate. Such bacteria are envisioned as being well-suited to the desulfurization of coal.
Other species, e.g., Pseudomonas putida, are capable of consuming organic sulfur molecules, converting them into water-soluble sulfur products. However, this process is merely incident to the utilization of the hydrocarbon portion of these molecules as a carbon source: valuable combustible hydrocarbons are lost. Moreover, MDS processes based on the use of these microorganisms most readily desulfurizes the same classes of organic sulfur molecules as are labile to HDS. Thus, although MDS does not involve exposing fossil fuels to the extreme conditions encountered in HDS, a significant amount of the fuel value of the coal or liquid petroleum so treated is lost, and the resultant fuel product often still requires post-combustion desulfurization. Microbial desulfurization technology is reviewed in Monticello and Finnerty (1985), 39 ANN. REV. MICROBIOL. 371-389 and Bhadra et al. (1987), 5 BIOTECH. ADV. 1-27. Hartdegan et al. (1984), 5 CHEM. ENG. PROGRESS 63-67 and Kilbane (1989), 7 TRENDS BIOTECHNOL. (NO. 4) 97-101 provide additional commentary on developments in the field.
A need remains to develop more effective methods for pre-combustion desulfurization. This need grows progressively more urgent as lower-grade, higher-sulfur fossil fuels are increasingly used, while concurrently the sulfur emissions standards set by regulatory authorities become ever more stringent.